The present invention is directed to silicon carbide cutting inserts of the type produced in accordance with teachings of U.S. patent application Ser. Nos. 167,019 and 167,196, by Dr. John M. Ohno, both filed July 9, 1980, both now abandoned, and assigned to the assignee of the present invention, the entire disclosures of which are hereby incorporated by reference.
Articles composed of materials having refractory characteristics such as hardness and resistance to erosion have many important uses in the fields of metal machining, such as boring, facing and the like. Representative materials are described in U.S. Pat. No. 2,938,807 to Anderson. More recently, a dispersion of a mass of diamond or other super-hard crystals is bonded together by a silicon containing bonding medium comprised of silicon carbide and a carbide of a metal component, such as that disclosed in U.S. Pat. No. 4,124,401 to Lee et al., and assigned to the assignee of the present invention. Such a polycrystalline diamond body is made by a high pressure-high temperature technique. The polycrystalline body may be sectioned into wafers, and the wafers brazed or otherwise bonded onto a pocket in a carbide substrate, one such construction being that of CARBO-PAX.RTM., which is made by the assignee of the present invention.
The above-mentioned technique for forming a cutting insert is very expensive and time consuming. Further, the inserts produced in accordance with this process are not indexable, thereby further increasing operating costs.
Recently, a new technique for economically and rapidly forming a polycrystalline diamond or cubic boron nitride body has been disclosed in the above-mentioned Ser. Nos. 167,019 and 167,196, both now abandanded. A straightforward technique (hereinafter referred to as the "press and treat" technique) for forming high quality cutting inserts is disclosed therein. Very briefly, the press and treat technique involves the preparation of a first or crystal dispersion of super-hard crystals such as diamond or cubic boron nitride crystals in carbon black and a second or a core dispersion of carbon black, carbon fiber and filler material (usually silicon carbide). The two dispersions are individually mixed with a small amount of temporary binder, such as paraffin, to lend a sufficient green strength to the two dispersions upon cold compaction thereof. After compacting the two dispersions together in a desired configuration, the compact is vacuum heated in the presence of silicon to burn off the paraffin and to allow the silicon to infiltrate both dispersions. Upon further heating, and without the need for the constant application of any type of pressure to the insert, the silicon reacts with the carbon black to form a .beta.-silicon carbide and a silicon matrix which bonds both dispersions both internally and to each other.
One such insert made according to the press and treat technique is described in co-pending U.S. patent application Ser. No. 226,603, filed Jan. 21, 1981, now abandoned, by Dr. John M. Ohno, and assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated by reference. The cutting insert produced in accordance with Ser. No. 226,603, now abandoned, includes a first layer containing super-hard crystals in a silicon carbide base, and a core layer. The composite is sandwiched between a metallic plate on top of the diamond surface and a metallic carbide base bonded to the core layer. Such insert performs well in high speed finish machining operations, such as the finish milling of Al-Si alloys.
In pending U.S. patent application Ser. No. 226,604, filed Jan. 21, 1981, now abandoned by Dr. John M. Ohno, and assigned to the assignee of the present invention, the entire disclosure of which is also incorporated by reference, a technique for producing a three layer bonded composite using the press and treat technique is disclosed. Specifically, an additional or intermediate super-hard crystal dispersion layer is disposed underneath the super-hard top layer around the periphery of the core, with the percentage of crystal material in the intermediate layer being less than that of the top layer which provides the cutting surface. The intermediate or peripherial layer is provided to hold the top layer rather than for wear-resistance. The insert is adapted to be bonded onto a tungsten carbide substrate and is designed primarily for the finish machining of abrasive nonferrous materials under typical conditions which include (a) a very high speed range such as 1,700-3,000 SFPM, (b) a small feed of 0.0025-0.003 inches per revolution and (c) a small depth of cut from 0.030 to 0.015 inches. Under such conditions, the insert provides efficient machining, high quality surfaces and a smear free finish with a significant cost savings.
However, the automobile industry now requires a type of insert which has much wider scope of capability, namely rough and interrupted heavy machining (up to 0.1 inch in cut depth) and high speed finish machining (1,500 SFPM) in continuous production lines. Typical new requirements are tabulated below in Table I.
TABLE I ______________________________________ SPEED FEED (SEPM) (in/rev) DEPTH OF CUT ______________________________________ Previous Require- 2000-3000 &lt;.009 &lt;.030 ments New Requirements 1500 .014 .050 (Examples) 1500 .010 .080 1000 .010 .100 ______________________________________
Castings of an Al-Si alloy provided at the receiving end of a production line typically have uneven wall thicknesses, interruptions due to openings in the design, voids and left over in-gate projecting from the surfaces. Therefore, the machining of such castings is not a simple and steady operation, but rather a high speed, interrupted and rough machining operation.
Further, for some applications, the auto industry requires such high speed, interrupted and rough machining operations using a triangularly shaped insert having a small nose radius. Due to the acuteness of the angle provided at the cutting point of the insert, such inserts are inherently weak in design, thus compounding the problem of rough machining. The use of such triangular inserts is necessary to machine corners of castings, for example.
Other configurations for the insert, such a square may be desired for facing of Al-Si 390 alloy castings, for example.